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Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by a ratio of well-defined natural numbers, and are therefore in violation of the law of definite proportions. They are most often solids that contain random defects, resulting in the deficiency of one element. Since the solid needs to be overall electrically neutral, the missing atom's charge needs to be compensated by a change in the charge for another atom in the solid (either by changing the oxidation state, or by replacing it with an atom of a different element with a different charge). For example, although ferrous oxide has an ideal (stoichiometric) formula FeO, crystallographic vacancies can reduce the amount of iron to somewhere around Fe_0.95O. To compensate for the charges of the missing Fe²⁺ ions, some of the atoms in the crystal have to be oxidized to Fe³⁺. Since the compositions of a non-stoichiometric compound can vary in a continuous manner, such a formula is often written as Fe_1-xO, where x is a small number (0.05 in the previous example) representing the deviation from the "ideal" formula.

Non-stoichiometric compounds are also known as berthollides (as opposed to the stoichiometric compounds or daltonides). The names come from Claude Louis Berthollet and John Dalton, respectively, who in the 19th century advocated rival theories of the composition of substances. Although Dalton "won" for the most part, it was later recognized that the law of definite proportions did have important exceptions.

Many non-stoichiometric compounds are important in solid state chemistry, and have applications in ceramics and as superconductors. For example, yttrium barium copper oxide, arguably the most notable high-temperature superconductor, is a non-stoichiometric solid with a formula represented by YBa₂Cu₃O_7−x. The critical temperature of the superconductor depends on the exact value of x.

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